Since I started to attend gym classes I've gained 5 kg of body mass. I had a BMI kinda low, but with some exercise and "bodybuilding" I'm no longer underweight. There's a big taboo about this kind of weight gaining, based on muscular mass, but let me tell that I've accomplished my goal without taking any steroids or supplements. Just pure exercise and fun. Maybe you guys, who are underweight, should try to attend some gym classes or so.

Also, it is normal for some people to be underweight. It's called ectomorphism and it's purely genetic. Sometimes being underweight is not a symptom of a disease; it's just how you've born: slender and lean.